1. Field of the Invention
The present invention relates to a surface-emitting laser device, a surface-emitting laser array including the same, an image forming apparatus including the surface-emitting laser array, an optical pickup unit including the surface-emitting laser device or the surface-emitting laser array, an optical transmitter module including the surface-emitting laser device or the surface-emitting laser array, an optical transmitter receiver module including the surface-emitting laser device or the surface-emitting laser array, an optical communication system including the surface-emitting laser device or the surface-emitting laser array, an optical scanner including the surface-emitting laser array, and an electrophotographic apparatus including the optical scanner.
2. Description of the Related Art
Surface-emitting laser devices (surface-emitting semiconductor laser devices) are semiconductor lasers that emit light in a direction perpendicular to a substrate. Since surface-emitting laser devices achieve high-performance characteristics with low cost compared with edge-emitting laser devices, surface-emitting laser devices are used for consumer applications such as a light source for optical communications such as an optical interconnection, a light source for optical pickups, and a light source for image forming apparatuses.
In particular, surface-emitting laser devices of 850 nm and 980 nm bands enjoy good confinement of carriers in an active layer. More specifically, surface-emitting laser devices of the 850 nm band employ a quantum well active layer formed of gallium arsenide (GaAs) and barrier layers and spacers (cladding layers) formed of aluminum gallium arsenide (AlGaAs).
Further, in surface-emitting laser devices of the 850 nm band, practical-level performance is realized because a current confinement structure using high-performance AlGaAs-system reflecting mirrors (such as semiconductor multilayer-film reflecting mirrors and semiconductor distributed Bragg reflectors [semiconductor DBRs]) and an Al oxide film can be adopted.
However, since the volume of the active layer is small in surface-emitting laser devices, surface-emitting laser devices are lower in light output than edge-emitting lasers, so as to be often required to increase output. In particular, as the wavelength becomes shorter, confinement of carriers in the active layer becomes poorer, thus causing problems such as inability to obtain high output and poor temperature characteristics.
Short-wavelength surface-emitting laser devices having an oscillation wavelength in the 780 nm band adopt a selectively oxidized AlAs layer as a current confinement structure. (See Non-Patent Document 1.) The surface-emitting laser device disclosed in Non-Patent Document 1 has a cavity (resonator) sandwiched between a lower reflecting mirror and a higher reflecting mirror, where the cavity has an active layer sandwiched between spacer layers.
The cavity has one oscillation wavelength's worth of thickness. The active layer has a quantum well structure of alternately stacked well layers of Al0.12Ga0.88As and barrier layers of Al0.3Ga0.7As. Further, the spacer layers are formed of Al0.6Ga0.4As. Further, the lower reflecting mirror has 40.5 stacked pairs of n-type Al0.3Ga0.7As high refractive index layers and n-type Al0.9Ga0.1As low refractive index layers. In this case, letting the oscillation wavelength of the surface-emitting laser device be λ, the film thickness of each of the high refractive index layers and low refractive index layers is λ/4.
Further, the upper reflecting mirror has 24 stacked pairs of p-type Al0.3Ga0.7As high refractive index layers and p-type Al0.9Ga0.1As low refractive index layers. In this case, the film thickness of each of the high refractive index layers and low refractive index layers is also λ/4.
Further, an AlAs selectively oxidized layer is provided λ/4 apart from the cavity in the upper reflecting mirror. A composition gradient layer that gradually changes in composition is provided between each adjacent two layers of each of the reflecting mirrors in order to reduce resistance.
The above-described layers such as the active and spacer layers are formed by MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy).
The surface-emitting laser device disclosed in Non-Patent Document 1 adopts a mesa shape. This mesa shape is formed by successively stacking the lower reflecting mirror, the (lower) spacer layer, the active layer, the (upper) spacer layer, and the upper reflecting mirror on a substrate and thereafter etching the upper reflecting mirror, the (upper) spacer layer, the active layer, and the (lower) spacer layer so as to reach the lower reflecting mirror by dry etching.
Once the mesa shape is formed, the edge surface of an AlAs layer to serve as the AlAs selectively oxidized layer is exposed. Accordingly, the AlAs layer is subjected to heat treatment in steam so as to convert AlAs into an insulator of AlxAsy, thereby forming a current confinement structure (oxide aperture) that limits the path of a device driving current to the central unoxidized AlAs region.
Thereafter, a p-side electrode is formed on the mesa except for a light exit part (metal aperture) at the top of the mesa, and an n-side electrode is formed on the bottom side of the substrate, thereby completing the surface-emitting laser device.
According to Non-Patent Document 1, an output of 3.4 mW, which is the maximum of a single mode in the 780 nm band, is obtained by optimizing the oxide aperture and the metal aperture.
However, an output of 7 mW has been reported in the 850 nm and 980 nm bands, showing that the surface-emitting laser device of the 780 nm band is inferior in output. One method of increasing this light output is to reduce an increase in the temperature of a light emission part.
As a method of suppressing an increase in the temperature of a light emission part, a configuration that reduces thermal resistance in a surface-emitting laser device having an oscillation wavelength of 850 nm has been proposed (Patent Document 1). This configuration employs AlAs, which is higher in thermal conductivity than AlGaAs, for a large proportion of low refractive index layers disposed in the lower part of a lower reflecting mirror.
Conventional AlGaAs is used for the low refractive index layers of the upper part of the lower reflecting mirror. If the etching surface reaches inside the lower reflecting mirror using AlAs at the time of forming the mesa shape, the exposed AlAs in the lower reflecting mirror is also oxidized at the time of forming an AlAs selectively oxidized layer by oxidation in the process subsequent to the etching, so that the device is insulated or has high resistance. Therefore, in order to avoid this, AlGaAs is used for the low refractive index layers of the upper part of the lower reflecting mirror.
That is, by providing AlGaAs lower in etching rate than AlAs on the upper side of the lower reflecting mirror, the etching surface is positioned inside AlGaAs on the upper side of the lower reflecting mirror.
Further, in surface-emitting laser devices of the 780 nm band, since active aluminum (Al) is added to the active layer, oxygen is captured during growth or processing, so that a nonradiative recombination center is formed in the active layer. This decreases light emission efficiency and reliability.
Therefore, in surface-emitting laser devices of a wavelength band shorter than 850 nm, a surface-emitting laser device of the 780 nm band that adopts an Al-free active region (quantum well active layers and their adjacent layers) in order to prevent formation of the nonradiative recombination center has been proposed (Patent Document 2). Specifically, GaAsP having tensile strain is used for quantum well active layers, GaInP having compressive strain is used for barrier layers, lattice-matching GaInP is used for spacer layers (between cladding layers and the first and third quantum well active layers), and AlGaInP is used for the cladding layers. Adoption of this configuration improves the reliability of the surface-emitting laser device.
Further, there has been proposed a surface-emitting laser device of the 780 nm band that, besides producing the effect due to the Al-free active region, uses GaInPAs having compressive strain for quantum well layers, uses lattice-matching GaInP or GaInP having tensile strain for barrier layers, and uses AlGaInP greater in Al composition than spacer layers for cladding layers in order to increase the gain of the active layer (Non-Patent Document 2). Compared with the structure of the surface-emitting laser device disclosed in Patent Document 1, this surface-emitting laser device, which has lattice-matching barrier layers and has a greater band gap than compressive strain composition, enjoys good carrier confinement.
However, there is a problem in that surface-emitting laser devices of short oscillation wavelengths are low in output.
Meanwhile, since surface-emitting lasers consume less power, have better mode stability, and are highly integrated more easily than edge-emitting lasers, their research and development have been active of late in expectation of application to the communication field and the image recording field.
In semiconductor lasers, the oscillation wavelength is determined by the band gap of the material of an active layer. In the visible range to the near infrared range, studies have been made of AlGaAs-system and (Al)GaInP-system materials. Of these, AlGaAs-system materials in particular have long been studied with many reports, and as reported in Non-Patent Document 1, a single-mode output characteristic of over 3 mW is realized with respect to surface-emitting laser devices. Products using the characteristic have already been commercially available.
However, in semiconductor lasers, Al is regarded as a cause of device degradation. Since AlGaAs-system materials inherently contain a cause of degradation, it is difficult to realize a highly reliable device with AlGaAs-system materials. On the other hand, it is relatively easy to realize a highly reliable device with GaInP-system and GaInAsP-system materials since Al is not contained in the active layer.
Meanwhile, surface-emitting laser devices have a structure where a cavity is vertically sandwiched between multilayer films each formed of two types of materials different in refractive index. Combinations of the two types of materials include AlxGa1-xAs/AlyGa1-yAs, (AlxGa1-x)0.5In0.5P/(AlxGa1-x)0.5In0.5P, and AlxGa1-xAs/(AlyGa1-y)0.5In0.5P (0≦x,y≦1, and x≠y). These material systems and compositions are suitably determined in accordance with the oscillation wavelength.
Further, surface-emitting laser devices have high device resistance for structural reasons so as to be characterized in that heat generated in the active layer is less likely to be emitted outside. That is, it is necessary to solve these problems in order to develop surface-emitting laser devices having good characteristics. In order to solve the former problem, a composition gradient layer is provided at each interface of the two types of materials forming each reflecting mirror. In order to solve the latter problem, materials having good thermal conductivity are employed.
With respect to the material conductivity, AlGaAs-system materials are better in thermal conductivity than AlGaInP-system materials if Al composition is the same. Non-Patent Document 3 reports a surface-emitting laser device using AlAs/Al0.25Ga0.75As.
However, in this reported case, (Al0.5Ga0.5)0.5In0.5P is employed as cavity spacers, and this material is joined to Al0.25Ga0.75As forming reflecting mirrors. However, the band discontinuity of the valence bands of these materials is relatively large, which may cause an increase in device resistance.
The case of joining AlGaAs-system reflecting mirrors and an AlGaInP-system cavity is disclosed in Non-Patent Document 4, but cannot avoid the same problem, either.
Further, in the case of successively causing crystal growth of an AlGaInP-system material and an AlGaAs-system material, it is necessary to switch the V-group material from a P material (such as PH3) to an As material (such as AsH3) after growth of the AlGaInP-system material. At this point, it is highly possible that a defect is introduced at their interface to cause various problems. In Patent Document 3, the possibility of the above-described increase in device resistance is low, but there is no description of the above-described P-containing material/As-containing material interface.
On the other hand, Patent Document 4 discloses a configuration where only an n-side reflecting mirror or each of a p-side reflecting mirror and the n-side reflecting mirror is formed of an AlGaInP-system material. However, since the AlGaInP-system material is inferior in thermal conductivity to the AlGaAs-system material, the temperature of the active layer is likely to increase during oscillation so as to degrade many characteristics.
Meanwhile, in image recording in electrophotography, image recording methods using a laser are widely used as image recording means for obtaining high-definition image quality. In the case of electrophotography, it is common to form a (sub scanning) latent image on a photosensitive drum by causing the drum to rotate while causing a laser to perform scanning (main scanning) in the axial direction of the drum using a polygon mirror.
Further, in the field of electrophotography, high-definition images and high-speed image recording are required. These may be realized by increasing laser output or the sensitivity of a photosensitive body while increasing the speed of main scanning and sub scanning. In the case of increasing image recording speed by this method, however, many problems such as development of a light source for high laser output or a highly sensitive photosensitive body, reinforcement of a housing that supports high-speed main and sub scanning, and development of a position control method at the time of high-speed scanning, thus necessitating expenditure of large amounts of money and time. Further, with respect to high-definition images, if the resolution of an image is doubled, the time required for each of main scanning and sub scanning is also doubled, so that the time required for outputting the image is quadrupled. Accordingly, it is also necessary to simultaneously achieve high-speed image outputting in order to realize high-definition images.
Another method for achieving high-speed image outputting may be to employ a multi-beam laser (multiple lasers). It is common to use multiple lasers in current high-speed output machines. Employment of multiple lasers expands the area in which a latent image is formed with a single main scan. In the case of using n lasers, the above-described latent image formation area is n times as large and the time required for image recording is 1/n times as much as in the case of using a single laser.
As such a case, a multi-beam semiconductor laser having multiple light emission sources in a single chip is proposed in Patent Document 5. However, with a configuration using an edge-emitting semiconductor laser as described in Patent Document 5, the number of beams is about four or at most eight for structural and cost reasons, so that it is impossible to support high-speed image outputting, which is expected to make progress in the future.
On the other hand, two-dimensional integration is easy for surface-emitting laser devices as described above. By modifying or varying the integration method, it is possible to make the actual beam pitch narrower and to integrate as many light-emitting devices as possible onto a single chip.
However, conventional surface-emitting laser devices have the problem of low output because carrier confinement is insufficient and heat generated in the active layer is less likely to be transferred outside.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2002-164621    [Patent Document 2] Japanese Laid-Open Patent Application No. 9-107153    [Patent Document 3] Japanese Laid-Open Patent Application No. 2004-281968    [Patent Document 4] Japanese Laid-Open Patent Application No. 2002-158406    [Patent Document 5] Japanese Laid-Open Patent Application No. 11-340570    [Non-Patent Document 1] Ueki, N. et al.; “Single-Transverse-Mode 3.4-mW Emission of Oxide-Confined 780-nm VCSEL's,” IEEE PHOTONICS TECHNOLOGY LETTERS, 11, No. 12, 1539-1541 (1999)    [Non-Patent Document 2] Tansu, N. et al.; “Low-Temperature Sensitive, Compressively Strained InGaAsP Active (λ=0.78-0.85 μm) Region Diode Lasers,” IEEE PHOTONICS TECHNOLOGY LETTERS, 12, No. 6, 603-605 (2000)    [Non-Patent Document 3] Schneider, R. P. Jr. et al.; “GaInAsP/AlGaInP-based near-IR (780 nm) vertical-cavity surface-emitting lasers,” ELECTRONICS LETTERS, 31, No. 7, 554-556 (1995)    [Non-Patent Document 4] Lott, J. A. et al.; “Partial top dielectric stack distributed Bragg reflectors for red vertical cavity surface emitting laser arrays,” IEEE PHOTONICS TECHNOLOGY LETTERS, 6, No. 12, 1397-1399 (1994)